How long will I be infectious after starting antibiotics? - Health questions - NHS Choices

How long will I be infectious after starting antibiotics?

After starting a course of antibiotics, the length of time you remain infectious can vary. However, you are usually no longer infectious 24 hours after starting antibiotics.

Factors affecting infectious period

Whether you’re still infectious can depend on several factors, such as those listed below.

The type of infection you have– different types of infection have different infectious periods, and some infections can be contagious before any symptoms appear.

The severity of the infection– if you have a severe infection, you may be infectious longer than if you have a mild infection. Severe infections may also need more than one course of antibiotics.

The length of time it takes for the antibiotics to start working– this can also vary; for example, it may be affected by how well your body absorbs the antibiotics and how much of the medication reaches the infection within your body.

The length of your course of antibiotics– this will depend on the infection you have, its severity, and how well you respond to treatment. Most courses of antibiotics last 5-7 days, although some urinary tract infections (UTIs) can be treated with one dose, and an infection like tuberculosis (TB) may require treatment over a long period of time.

Whether you are taking other medications as well as your antibiotics – other medications may interact with your antibiotics. Your GP will take this into account when prescribing your medication.

GP advice

Ask your GP if you are concerned about how long you will be infectious for after starting a course of antibiotics.

Antibiotics work by killing bacteria or stopping them multiplying, which allows your body’s immune system to get rid of them.

Your GP may prescribe antibiotics to treat a bacterial infection. However, they are not prescribed for viral infections, because they do not work on viruses.

It's important to finish your course of antibiotics, even if your symptoms improve and you're feeling better. This will ensure that all of the bacteria are killed. Failing to finish the course may result in the infection returning. Alternatively, the bacteria may become resistant to that type of antibiotic, which could mean they won't work if you need to take them again in the future.